1. Field of the Invention
This invention relates to a power supply regenerating circuit in an induction motor drive circuit which performs drive control and regenerative control.
2. Description of Related Art
There are cases in which an induction motor is repeatedly accelerated and decelerated due to the characteristic of a driven load, at which time the load torque varies positively and negatively.
Though various control means are used in order to process the rotational energy at the time of deceleration, a regenerative control circuit is widely employed. The regenerative control circuit converts the rotational energy into power by electrical braking means and transmits the power to the power supply on the motor side.
FIG. 7 is a circuit diagram of a regenerative control circuit of this type. When an induction motor is driven, three-phase power is converted into a direct current and the power is supplied to the induction motor as a predetermined AC power by an inverter. At the time of power regeneration, the power from the AC motor is rectified. It is then converted into AC power by the regenerative control circuit, which is composed of transistors and a thyristor bridge, to carry out power regeneration.
In FIG. 7, a three-phase full-wave rectifier is formed by diodes D1-D6. Power that has been converted into direct current is supplied to a transistor inverter, which is composed of transistors Tr11-Tr16, via a smoothing circuit of a capacitor C3. In response to a command signal from a control circuit (not shown). Voltage and frequency are controlled and the AC power resulting from the conversion is supplied to an induction motor M, whereby the motor is run at a predetermined rotational velocity.
When regenerative control is performed, MCC1-MCC3 are turned off, the induced voltage of the induction motor M produced at deceleration is converted into direct current by a three-phase rectifier circuit composed of diodes D11-D16. The direct current is converted into AC power having the same frequency as the supplied power by the regenerative control circuit, which comprises a thyristor bridge SCR1-SCR6, via the smoothing circuit of the capacitor C3. This AC power is then regenerated in the power supply, thus achieving regenerative control.
In the regenerative control circuit of the above-described type, the induced power of the induction motor is converted into direct current and made smooth. Consequently, a large circuit flows into the thyristor bridge SCR1-SCR6 at onetime during the AC regeneration period. A PWM control circuit, for example, is used as a complicated control circuit to limit this current.
Furthermore, the control signal, when DC power is converted into AC power at the time of the regenerative operation, performs a frequency setting operation by a manual changeover. However, if the set frequency and the power supply frequency differ, an abnormal current flows and, hence, there is the danger that the transistor bridge will burn out.